FIG. 1 illustrates a prior art equipment configuration used to measure the echo return loss for one terminal of a communication link. The communication link has a near-end terminal 8 comprising a telephone 2, a four-to-two wire hybrid circuit 3, and an echo canceller circuit 4. The far-end terminal is similarly configured but, for simplicity, is represented by telephone 5, alone. During a conversation between the near-end and far-end users, the far end signal, which contains both the far-end user's speech and incidental background noise, enters the near-end terminal 8 as signal X at node 9.
The far-end signal is provided to the four-to-two wire hybrid circuit 3 and then to near-end telephone 2. Due to the unavoidable non-linearities present in the hybrid circuit 3, some portion of the far-end signal power is coupled onto the output 7 of the hybrid circuit 3 as an echo. A composite signal Y exists at node 7 containing the echo signal and the combined speech of the near-end user and any incidental background noise from the near-end user's environment. A filter having a filter length period selected and designed to be longer than the hybrid dispersion time is used prior to power level measurements at 7 to allow the echo canceller 4 to operate properly.
Echo canceller 4 synthesizes the expected value of the echo signal and subtracts this value from the composite signal Y existing at node 7. The resulting difference signal, existing at node 14, is intended to contain only the near-end signal originating from telephone 2. Ultimately, difference signal, e, is provided to the far-end telephone 5.
Prior art methods of measuring the echo return loss rely on disconnecting the far-end telephone 5 from the near-end terminal 8 and connecting a test signal generator 6 in a manner illustrated as switch 10 of FIG. 1. Test signal generator 6 injects a conditioned and reproducible test signal into the near-end terminal 8 at node 9, where the signal power from the far-end terminal would normally exist. This test signal may be synthesized pseudo-noise, pseudo-white noise, or tones of a constant power level.
A measurement of the test signal power, X, at node 9 is made. Additionally, the power level of the composite signal Y, comprised of the coupled echo signal and any signal generated by the near-end telephone 2, is measured at node 7. The test is made when little to no signal is being generated at near end telephone 2. Assuming the signal power of any signal generated by the near-end telephone is very small in comparison to the coupled echo signal power, the ratio of the measured test signal power X to the measured power level Y provides an estimate of the echo return loss for the near-end terminal 8.
The shortcoming of the prior art method is that the echo return loss may not be measured dynamically during the course of a telephone conversation. Rather, the echo return loss may only be measured by disconnecting the far-end terminal from the near-end terminal 8 and injecting a conditioned and reproducible test signal into the near-end terminal 8.